DRUG CALCULATIONS - Christchurch Hospital
DRUG CALCULATIONS
When calculating how much of a drug is required, working with the formula helps the accuracy of the calculation.
Always remember this formula:
What you want X Quantity it comes in
What you have
1
It's OK to use a calculator!
1. A patient requires 4 mg of Morphine IVI. Morphine is available as 10mg/ml. How many mls will you draw up?
2. Gentamycin 360 mg is prescribed. Gentamycin is available as 80mg/2ml. How many mls will you draw up?
3. Haloperidol 3 mg IVI is charted. Haloperidol is available as 5mg/ml. How many mls is required?
4. Frusemide 70mg IVI is charted. Stock dose is 20mg/ml. How many ml would you give?
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C:\Documents and Settings\neilh\Local Settings\Temporary Internet Files\Content.Outlook\BA80LT24\Clinical calcs REVIEW SESSION.doc Prepared by: Janet Tweedy and Deb Mason, Nurse Educators, Department of Nursing, Christchurch Hospital, May 2000. Canterbury Health Ltd. Last updated June 2000.
METRIC CONVERSIONS
Metric weights and measurements involve a step-by-step conversion from one unit to another.
With weight we often convert to smaller (and more numerous units) thus:
Kg
gm
mg
mcg
grams milligrams micrograms
Each of these steps involves the heavier unit being multiplied by 1000 to bring up the number of the smaller units for the same weight.
kg
g 1kg x 1000 = 1000g
g
mg 1g x 1000 = 1000mg
mg
mcg 1mg x 1000 = 1000mcg
When we multiply by 1000 we move the "decimal point" three places to the right
0.5g == ?? mg
0.5 x 1000 == 500mg 1 place = 5 2 places = 50 3 places = 500
If we are converting from a lighter unit to a heavier unit we move the decimal point three places to the left for each conversion. Another way of putting it is we divide by 1000.
mcg
mg 1000mcg 1000 = 1mg
mg
gm 1000mg 1000 = 1gm
gm
kg 1000gm 1000 = 1kg
500mg = ?? gm
500 1000 = 0.5mg 1 place = 50 2 places = 5 3 places = 0.5
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C:\Documents and Settings\neilh\Local Settings\Temporary Internet Files\Content.Outlook\BA80LT24\Clinical calcs REVIEW SESSION.doc Prepared by: Janet Tweedy and Deb Mason, Nurse Educators, Department of Nursing, Christchurch Hospital, May 2000. Canterbury Health Ltd. Last updated June 2000.
Try these:
5. Atropine 0.6 mg = ?mcg
0.6 x 1000 = 600mcg
6. 0.01gm = ? mg
0.01 x 1000 = 10 mg
7. Gentamycin 360mg = ?gm
360 1000 = 0.36gm
8. Digoxin 125mcg = ? mg
125 1000 = 0.125mg
If we are moving across two conversions we repeat the process twice. For example from grams to micrograms:
3gm converted to micrograms Step one:
= 3 x 1000 = 3000mg Step two:
3000mg x 1000 = 3,000,000mcg
9. Augmentin 1.2gm = ?mcg
1.2 x 1000 = 1200mg 1200 x 1000 = 1,200,000mcg
10. Digoxin 125mcg = ? gm
125 1000 = 0.125mg 0.125 1000 = 0.000125gm
Memory Tip: "Heavy to light ? three places to the right"
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C:\Documents and Settings\neilh\Local Settings\Temporary Internet Files\Content.Outlook\BA80LT24\Clinical calcs REVIEW SESSION.doc Prepared by: Janet Tweedy and Deb Mason, Nurse Educators, Department of Nursing, Christchurch Hospital, May 2000. Canterbury Health Ltd. Last updated June 2000.
Try these: Convert 11. 1.2 mg to mcg 12. 1.3g to mg 13. 500mcg to mg 14. 0.04 mg to mcg 15. 20mcg to mg 16. 600mcg to g 17. 2g to mcg
When we are converting volumes the process is the same.
Litres 1 Litre
0.25L 375ml
ml 1000ml
250ml 0.375L
Try these: Convert 18. 450ml to L 19. 64 ml to L 20. 4.3L to ml
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C:\Documents and Settings\neilh\Local Settings\Temporary Internet Files\Content.Outlook\BA80LT24\Clinical calcs REVIEW SESSION.doc Prepared by: Janet Tweedy and Deb Mason, Nurse Educators, Department of Nursing, Christchurch Hospital, May 2000. Canterbury Health Ltd. Last updated June 2000.
MG/KG DOSE CONVERSION Often we need to check the dose of a drug or solution that is based on the patient's weight. We multiply the prescribed mg dose by the kg weight of the patient
Prescribed dose - 25mg per kg Patient's weight - 66kg What is the dose required?
25mg x 66kg = 1650mg = 1.65g
21. Prescribed dose ? 50 mg/kg Patient's weight ? 79 kg What is the dose required?
22. The patient is charted 15mg/kg/day. The patient weighs 75kg. a) How much is the total dose per 24 hours?
b) How much will the patient receive every 8 hours?
ROUNDING OFF the decimal point Rounding up if greater than 5, round up Rounding down if less than 5, round down
eg 166.66 = 167 eg 33. 33 = 33
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C:\Documents and Settings\neilh\Local Settings\Temporary Internet Files\Content.Outlook\BA80LT24\Clinical calcs REVIEW SESSION.doc Prepared by: Janet Tweedy and Deb Mason, Nurse Educators, Department of Nursing, Christchurch Hospital, May 2000. Canterbury Health Ltd. Last updated June 2000.
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